Buy a Raspberry Pi 4 or Raspberry Pi 5 kit. They run anywhere from about $140 to $180 depending on whether it's a Pi 4 or Pi 5 and how much onboard memory it has, 4 GB or 8GB. I recommend the Pi 5 8GB kit as it's the most capable. All you need after that is a keyboard, mouse and monitor and you'll have a decently capable little (as in very small) computer for very little outlay.
All Raspberry Pi computers run Debian v12 (Bookworm), 32 bit or 64 bit depending on your preference. The OS is typically loaded on a 32GB MicroSD card, though you can load and boot it from a USB SSD if you want to get fancy. There are inexpensive add-on boards that will allow connecting up to two NVME SSDs if you'd rather go that route.
Dual 4k video micro HDMI ports, gigabit ethernet, four USB ports (two USB 2, two USB 3), and other neat connectivity.
You can load and reload the OS as often as you wish while experimenting. The Raspberry Pi 4 and Pi 5 are not limited to Debian Bookworm, either. Other flavors of Linux and various other desktops can be loaded and experimented with to your heart's content. Load the different OSes on separate MicroSD cards and just swap them out when you want to boot one to work with.
Canakit and Vilros are both authorized resellers of the Raspberry Pi computers. You can find their stuff on Amazon.